Because IP telephony devices are configured to use and rely on a DHCP server for IP configuration information, you must deploy DHCP servers in a redundant fashion. At least two DHCP servers should be deployed within the telephony network such that, if one of the servers fails, the other can continue to answer DHCP client requests.
The above is Cisco recommendation on DHCP server.
But as we know is that DHCP is single point of failure we can not deploy two of them, otherwise phone may get duplicate IP address since the so called redundant dhcp servers may designed to have same scope.
I also copy the following from another cisco link to prove my concept which is only 1 dhcp allowed.
No - Redundant DHCP is possible in many ways depending on what DHCP server you run:
For Windows servers you have two main options
1) Clustering (typically requiring specific hardware)
2) Create two scopes (identical) on two seperate servers. On server A, create an exclusion that covers the first half of the scope. On Server B, create an exclusion that covers the second half of the scope. This prevents one server from allocating addresses from the other server; and also stops one server from sending out NAKs to DHCP requests/renews intended for the other server.
I've used this second setup lots of times with no problems.
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Assuming there are phones over the wan and with the ip helper-address to point to these two servers, meantime there are ip phones in the same vlan as dhcp server which use broadcast to get the ip address.
My question is:
1.from remote site , if the first listed dhcp server is full, will the phone send the dhcp to the 2nd server?
2.At local lan, if the broadcast being received by two servers at the same time, and one server address is used out which may send no address available kind of message to the phone. Meantime the other server still has addresses available, will this cause problem?
I would think that it would go to the next server as long as you have both listed as IP helper addresses on the Router interface. On large LAN environments packed full of VLANs, that is pretty standard operation.
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The short answer is that you don't.... That isn't entirely true while at
the same time it kind of is, but for the most part you don't configure
the softkeys. You enable or disable them via TCL. Here is the long
answer. Be sure to read the whole thing or e...
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Phone Models 802.1X support? 802.1x flavor Addtl Comment EAP-MD5 EAP-TLS
Cisco 3905 Y Y N Cisco 6911 Y Y N Cisco ...